Transposable elements were discovered in maize by Barbara McClintock as the cause of several unstable mutations. An example of a nonautonomous element is Ds, the transposition of which requires the presence of the autonomous Ac element in the genome.
Bacterial insertion sequence elements were the first transposable elements isolated molecularly. There are many different types of IS elements in E. coli strains, and they are usually present in at least several copies. Composite transposons contain IS elements flanking one or more genes, such as genes conferring resistance to antibiotics. Transposons with resistance genes can insert into plasmids and are then transferred by conjugation to nonresistant bacteria.
There are two major groups of transposable elements in eukaryotes: class 1 elements (retrotransposons) and class 2 elements (DNA transposons). The P element was the first eukaryotic class 2 DNA transposon to be isolated molecularly. It was isolated from unstable mutations in Drosophila that were induced by hybrid dysgenesis. P elements have been developed into vectors for the introduction of foreign DNA into Drosophila germ cells.
Ac, Ds, and P are examples of DNA transposons, so named because the transposition intermediate is the DNA element itself. Autonomous elements such as Ac encode a transposase that binds to the ends of autonomous and nonautonomous elements and catalyzes excision of the element from the donor site and reinsertion into a new target site elsewhere in the genome.
Retrotransposons were first molecularly isolated from yeast mutants, and their resemblance to retroviruses was immediately apparent. Retrotransposons are class 1 elements, as are all transposable elements that use RNA as their transposition intermediate.
The active transposable elements isolated from such model organisms as yeast, Drosophila, E. coli, and maize constitute a very small fraction of all the transposable elements in the genome. DNA sequencing of whole genomes, including the human genome, has led to the remarkable finding that almost half of the human genome is derived from transposable elements. Despite having so many transposable elements, eukaryotic genomes are extremely stable as transposition is relatively rare because of two factors. First, most of the transposable elements in eukaryotic genomes cannot move because inactivating mutations prevent the production of normal transposase and reverse transcriptase. Second, expression of the vast majority of the remaining elements is silenced by the RNAi pathway in plants and C. elegans and the piRNA pathway in animals. Silencing of transposable elements depends on the ability of the host to detect new insertions in the genome and generate small noncoding RNAs that guide protein complexes to degrade complementary transposon-
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